Formation of silicon centered spirocyclic compounds: Reaction of N-heterocyclic stable silylene with benzoylpyridine, diisopropyl azodicarboxylate, and 1,2-diphenylhydrazine

Article abstract of DOI:10.1021/ic102566c

Three silicon centered spirocyclic compounds 1-3, possessing silicon fused six- and five-membered rings have been prepared by the reaction of NHSi (L) [L = CH{(C=CH₂)(CMe)(2,6-iPr₂C₆H₃N) ₂}Si] with be

Full text of DOI:10.1021/ic102566c

ARTICLE
pubs.acs.org/IC
Formation of Silicon Centered Spirocyclic Compounds: Reaction of
N-Heterocyclic Stable Silylene with Benzoylpyridine, Diisopropyl
Azodicarboxylate, and 1,2-Diphenylhydrazine
Ramachandran Azhakar,† Sankaranarayana Pillai Sarish,† Gaꢀsper Tavꢀcar,† Herbert W. Roesky,*,† Jakob Hey,†
Dietmar Stalke,*,† and Debasis Koley‡
†Institut f€ur Anorganische Chemie, Universit€at G€ottingen, Tammannstrasse 4, 37077 G€ottingen, Germany
‡Department of Chemical Sciences, Indian Institute of Science Education and Research-Kolkata, Mohanpur-741252, India
S Supporting Information
b
ABSTRACT: Three silicon centered spirocyclic compounds
1-3, possessing silicon fused six- and five-membered rings have
been prepared by the reaction of NHSi (L) [L = CH{(C=CH2)-
(CMe)(2,6-iPr2C6H3N)2}Si] with benzoylpyridine, diisopro-
pyl azodicarboxylate, and 1,2-diphenylhydrazine, respectively,
in a 1:1 ratio. The three spirocyclic compounds (1- 3) were
obtained by three different pathways. The reaction of L with
benzoylpyridine leads to the activation of the pyridine ring, and
dearomatization occurred. Treatment of diisopropyl azodicar-
boxylate with L favors a [1 þ 4]- rather than a [1 þ 2]-
cycloaddition product, and the azo compound was converted to hydrazone derivative. Finally the reaction of 1,2-diphenylhydrazine
and L results in the elimination of hydrogen by activating one of the C-H bonds present in the phenyl ring. All three complexes 1-
3 were characterized by single crystal X-ray structural analysis, NMR spectroscopy, EI-MS spectrometry, and elemental analysis. In
addition the optimized structures of probable products and possible intermediates were investigated using density functional theory
(DFT) calculations.
’ INTRODUCTION
[L = CH{(C=CH2)(CMe)(2,6-iPr2C6H3N)2}Si],3c we reported
N-H bond activation of ammonia,9b hydrazines,9c and C-H as
well as C-F bond activation of fluoroarenes.18 To explore the
reactivity of L further, we carried out the reactions of L with
benzoylpyridine, diisopropyl azodicarboxylate, and 1,2-diphenyl-
hydrazine. The silicon atom which is already part of the six-
membered ring system based on L, acts as a reaction center to
form new five-membered ring compounds with the respective
reactant keeping the six-membered ring intact and leading to the
spirocyclic compounds (1-3). In all three compounds, the six-
and five-membered rings are fused by the silicon center, and the
rings are arranged nearly orthogonal to each other. Interestingly
the reaction with benzoylpyridine leads to the activation of the
pyridine ring, and dearomatization emerged out of the benzoyl-
pyridine. Recently the pyridine activation-dearomatization reac-
tion has materialized as a cost-effective approach to asymmetric
synthesis in organic chemistry.19 The reaction of L with diiso-
propyl azodicarboxylate favors a [1 þ 4]- rather than a [1 þ 2]-
cycloaddition species. Here the hydrazone derivative is formed
from the azo compound. Upon reaction of L with 1,2-diphenyl-
hydrazine, the H atom present in one of the -NH units, and the
Silylenes are divalent silicon compounds and are extremely
reactive species, which are considered to be the silicon analogues
of carbenes.1 In 1994, the first stable N-heterocyclic silylene
(NHSi) was reported by Denk et al.;2 after that many stable
N-heterocyclic silylenes (NHSis)1b-d,3 and a stable N-hetero-
cyclic carbene (NHC) stabilized dichlorosilylene have been
reported.4 The reactivity of NHSis is quite comparable with that
of the N-heterocyclic carbenes (NHCs), where the latter led to
rapid research activities and to many applications in chemistry.5,6
However, the reactivity patterns of transition metal complexes of
NHCs are relatively different from those of the NHSis.6m In
general, silylenes have two non bonding electrons like the highest
occupied molecular orbital (HOMO) which possesses nucleo-
philic character and an empty p-orbital as the lowest unoccupied
molecular orbital (LUMO) which causes the electrophilic char-
acter. So they exhibit both nucleophilic and electrophilic reactive
sites at the same silicon atom, quantified by charge density
determination.4 Silylenes are considered to possess an ambiphilic
character, behave as Lewis acids as well as Lewis bases.7 Hence
there is intense research activity carried out in the chemistry of
NHSis. (Insertion: C-H,8 N-H,9 O-H,10 S-H,9a P-P,11 N-
Si,12 C-X (Cl, Br, I),13 Si-Cl,13 P-H,14 As-H,14 addition,9c,15
metal complexes,16 Lewis acids17). Utilizing the stable NHSi (L)
Received: December 24, 2010
Published: March 10, 2011
r
2011 American Chemical Society
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Scheme 1. Synthesis of 1, 2, and 3
H atom present in the o-position of one of the phenyl rings which
is attached with the uncleaved NH unit in 1,2-diphenylhydrazine
gets activated with the elimination of hydrogen.
pyridine product 1. It is soluble in n-hexane, n-pentane, toluene,
and benzene. Moreover it is stable both in the solid state as well as
in solution for a long time without any decomposition under an
inert gas atmosphere. The 29Si NMR spectrum of 1 shows a
single resonance at δ = -53.95 ppm, which is upfield shifted
compared to L (δ 88.4 ppm) because in 1 the silicon atom is
’ RESULTS AND DISCUSSION
1
more shielded. The γ-CH proton for compound 1 in the H
Compounds 1, 2, and 3 were obtained in good yield upon
treatment of L with benzoylpyridine, diisopropyl azodicarbox-
ylate, and 1,2-diphenylhydrazine in 1:1 ratio, respectively, as
represented in Scheme 1. They were fully characterized by NMR
spectroscopy, EI-MS, and elemental analysis. Furthermore, all
structures were unequivocally confirmed by single crystal X-ray
structural analyses.
The reaction of L with an equimolar amount of benzoylpyr-
idine in n-hexane at room temperature proceeds rapidly with the
formation of burgundy red colored solution of the dearomatized
NMR spectrum observed at δ = 5.31 ppm is upfield shifted
compared to that of L (δ 5.44 ppm). The NCCH2 protons in 1
resonate at 3.34 and 3.96 ppm (δ for L = 3.32 and 3.91 ppm).
The concomitant of the dearomatized pyridine ring has been
clearly established in 1H NMR of the upfield shift of hydrogen
atoms present in the pyridine ring. The dearomatized pyridine
ring shows multiplets in the region of 4.86-4.93(1H), 5.45-
1
5.53(1H), and 6.13-6.21(2H), respectively, in its H NMR
spectrum. These values indicate the possible dearomatized
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Table 1. Molecular Structurea and Bond Parameters for the Compounds 1, 2, and 3
a Hydrogen atoms in their respective molecular structure have been omitted for clarity except on backbone carbon atom.
pyridine ring product which is reported in the literature.19b
Moreover 1 shows its molecular ion peak in its mass spectrum
at m/z 627.4.
angle between N1-Si1-N2 is 105.59(6)°.There is a shortening
of bonds observed between the Si atom and the nitrogen atoms
of the supporting ligand. The average bond distance between Si-
Nav in 1 is 1.7065(11) Å, whereas in L it is 1.7345(10) Å. There is
appreciable change in the N-Si-N bite angle at the silicon atom
with the backbone ligand. In 1 it is 105.59(6)°, whereas in L it is
99.317(54)°. The bond lengths of Si1-N3(py) and Si1-O1 are
1.7468(12) and 1.6345(10) Å, indicating single bond
character.20 The bond distances between adjacent carbon atoms
in the dearomatized six-membered pyridine ring (N3, C31, C35,
C34, C33, C32) of compound 1 possess both single and double
bond character, the bond distances between C31-C35 and
C34-C33 are 1.4440(19) and 1.438(2) Å indicating C-C
single bonds, and the bond distances between C35-C34 and
C33-C32 are 1.346(2) and 1.342(2) Å indicating CdC double
bonds. The bond distances of C32-N3 and N3-C31 are
The dearomatized pyridine ring product 1 was unambiguously
established by single crystal X-ray structural analysis. 1 crystal-
lizes in the triclinic space group P1 and the molecular structure is
shown in Entry 1 of Table 1. We propose that the reaction of L
with benzoylpyridine followed the [1 þ 4]-cycloaddition reac-
tion as represented in Scheme 2. The silicon atom is in a distorted
tetrahedral geometry, made up from three nitrogen atoms and a
single oxygen atom. Among the three nitrogen atoms, one
originates from the dearomatized pyridine ring of the benzoyl-
pyridine and the two others from the chelating ligand. The silicon
atom is shifted out of the plane defined by N1, N2, C3, C4, and
C2 significantly by 0.28 Å. The angle included by O1-Si1-N3 is
92.30(5)° and differs from ideal tetrahedral symmetry while the
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Scheme 2. Proposed Mechanism for the Formation of 1
Scheme 3. Proposed Mechanism for the Formation of 2
1.3793(18) and 1.4288(17) Å. The distance between C30-C31
is 1.3643(19) Å indicating CdC double bond character.21 These
results substantiate the depicted Lewis formula of 1 (Scheme 1).
Similar to 1, compound 2 also contains a newly formed
five-membered ring after the reaction of diisopropyl azodicar-
boxylate with L. We presume that the reaction also proceeds
via [1 þ 4]-cycloaddition like that in 1 rather than a [1 þ 2]-
cycloaddition as represented in the Scheme 3. Unlike 1,
compound 2 is insoluble in n-hexane and n-pentane but
soluble in benzene and toluene. It is stable in both the solid
and the solution state under an inert atmosphere. Like 1, the 29Si
NMR spectrum of compound 2 shows upfield resonance δ at -
53.94. In the 1H NMR spectrum the γ-CH proton for compound
2 is observed at δ = 5.27 ppm which is upfield shifted compared
to that of L. The NCCH2 protons resonate at δ 3.34 and δ 3.89
ppm. Compound 2 shows its molecular ion peak in its EI-MS at
m/z 646.3.
Entry 2 of Table 1. Here the azo compound (-NdN-C-) has
been converted to the hydrazone derivative (-N-NdC-). In
2, the silicon atom is coordinated in a distorted tetrahedral
geometry comprising three nitrogen atoms (two from the
supporting ligand and one from the hydrazone moiety) and
one oxygen atom. The angle defined by O10-Si1-N4
(89.49(5)°) is slightly shorter than the corresponding angle in
compound 1 while N1-Si1-N2 includes an angle of
107.39(6)°. As in 1, there is Si-N bond shortening observed
toward the supporting ligand. The silicon atom is shifted out of
the plane defined by the N2C3-plane of the six-membered
SiN2C3 ring by 0.35 Å. The N-N [N(4)-N(5) 1.4299(15)
Å] and C-O [C(33)-O(10) 1.3553(16) Å] bond distances
present in the newly formed five-membered ring of 2 are quite
comparable to those values as reported in the literature.22 The
distance between C(33)-N(5) is 1.2843(18) Å indicating CdN
double bond character.
Compound 2 crystallizes in the orthorhombic space group
The reaction of L with an equimolar amount of 1,2-diphenyl-
Pca21. The molecular structure for compound 2 is shown in
hydrazine in n-hexane at room temperature leads to 3 in almost
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Scheme 4. Proposed Mechanism for the Formation of 3
quantitative yield with elimination of hydrogen. We assume that
initially one of the N-H bonds present in 1,2-diphenylhydrazine
gets activated and then the C-H bond present in o-position of
one of the phenyl rings which is attached with the uncleaved NH
unit in 1,2-diphenylhydrazine gets activated with elimination of
hydrogen as represented in Scheme 4. Like 1, compound 3 is
soluble in n-hexane, n-pentane, benzene, and toluene. In addition
it is stable in both the solid and the solution state under an inert
atmosphere. The 29Si NMR spectrum of compound 3 shows a
resonance at δ = -39.35 ppm, which is upfield shifted similar to
those of 1 and 2. In the 1H NMR spectrum the γ-CH proton for
compound 3 resonates at δ = 5.40 ppm, which is upfield shifted
compared to that of L. The N-H proton shows a broad singlet at
4.68 ppm and the NCCH2 protons exhibit sharp singlets at 3.33
and 4.02 ppm. The molecular ion peak of compound 3 is
observed at m/z 626.4.
Compound 3 crystallizes in the triclinic space group P1. The
molecular structure of compound 3 is shown in Entry 3 of
Table 1. In 3, the silicon atom exhibits a distorted tetrahedral
geometry, made up of three nitrogen atoms (one is derived from
the nitrogen atom from the hydrazine part and the other two
from the supporting ligand) and one carbon atom from the
phenyl ring. Like in 1 and 2, the SiN2C3 six-membered ring in 3 is
dislocated from the plane defined by N2C3 significantly (0.41 Å).
The angle N4-Si1-C30 is 90.48(7)°, while the observed angle
of N1-Si1-N2 is 102.80(7)° which is the smallest among the
Figure 1. BP86/SVP optimized structures of the complexes with
hydrogens removed for clarity. Color code: N blue, O red, Si cyan, C
gray, H black.
three compounds. The bond lengths of Si1-N4 and Si1-C30
are 1.7600(14) and 1.8400(16) Å, indicating single bond
character.20 The bond distances of the phenyl ring (between
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Table 2. Crystal and Structure Refinement Parameters for Compounds 1, 2, and 3
parameters
1
2
3
empirical formula
formula weight
temperature/K
wavelength/Å
crystal system
C
41 H49 N3 O Si
C37 H54 N4 O4 Si
646.93
C41 H50 N4 Si
626.94
627.92
100 (2)
99 (2)
80 (2)
0.71073
0.71073
0.71073
triclinic
P1
triclinic
orthorhombic
Pca21
space group
P1
unit cell dimensions
a = 11.2420 (7) Å
b = 11.5555 (7) Å
c = 14.8291 (9) Å
R = 78.5770 (10)°
β = 74.8600 (10)°
γ = 71.9120 (10)°
1753.22 (19) Å3, 2
1.189 mg/m3
a = 16.6931 (14) Å
b = 12.1192 (10) Å
c = 17.9703 (15) Å
R = 90°
β = 90°
γ = 90°
3635.5 (5) Å3, 4
1.182 mg/m3
a = 10.9890 (17) Å
b = 12.566 (2) Å
c = 12.909 (2) Å
R = 92.357(3)°
β = 93.727(3)°
γ = 90.731(4)°
1777.1 (5) Å3, 2
1.172 mg/m3
0.100 mm-1
volume, Z
density (calculated)
absorption coefficient
F (000)
0.103 mm-1
0.108 mm-1
676
1400
676
Crystal size
0.12 ꢀ 0.08 ꢀ 0.06 mm2
1.43 to 27.18°
-14 e h e 14
-14 ek e 14
-19 e l e 19
43551
0.15 ꢀ 0.12 ꢀ 0.09 mm2
1.68 to 27.13°
-21 e h e 21
-15 ek e 15
-23 e l e 23
66985
0.20 ꢀ 0.12 ꢀ 0.06 mm2
θ range for data collection
limiting indices
1.58 to 25.35°
-13 e h e 12
-10 e k e 15
-15 e l e 15
reflections collected
29684
independent reflections
completeness to θ
7717 (Rint = 0.0312)
98.9% (θ = 27.18)
full-matrix least-squares on F2
7717/0/424
8037 (Rint = 0.0382)
99.9% (θ = 27.13)
full-matrix least-squares on F2
8037/1/428
6480 (Rint = 0.0487)
99.6% (θ = 25.35)
full-matrix least-squares on F2
6480/23/427
refinement method
data/restraints/parameters
goodness-of-fit on F2
final R indices [I > 2σ (I)]
R indices (all data)
1.021
1.057
1.030
R1 = 0.0399, wR2 = 0.0924
R1 = 0.0503, wR2 = 0.0977
0.300 and -0.347 e Å -3
R1 = 0.0293, wR2 = 0.0719
R1 = 0.0322, wR2 = 0.0737
0.219 and -0.236 e Å-3
R1 = 0.0405, wR2 = 0.0895
R1 = 0.0584, wR2 = 0.0979
0.255 and -0.323 e Å-3
largest diff. peak and hole
adjacent carbon atoms C30 to C35) are in the order of 1.38 to
1.40 Å, indicating complete delocalization of π-electron density
within the ring, retaining its aromatic character. The distance
between N3-N4 is 1.4425(18) Å, indicating N-N bond length
which is quite comparable to the N-N bond distances of other
hydrazine or hydrazone derivatives which are reported in
literature.22
In all three reactions, new five-membered rings have been
formed (SiNC2O in 1, SiN2CO in 2, and SiN2C2 in 3). The five-
membered rings are nearly orthogonal to the SiN2C3 six-mem-
bered ring of L. The dihedral angles between the planes of six-
and five-membered rings are 89.18(3), 88.79(4), and 88.11(4)°
in compounds 1, 2, and 3, respectively.
activates one of the N-H bonds to furnish the intermediate 3I
(Scheme 4 and Figure 1). Closer inspection of the DFT optimized
3I reveals that the o-hydrogen of one of the phenyl rings is nearer to
silicon than the amine hydrogen (Si-H (from Ph) = 2.850 Å vs
Si-H (from -NH-) = 3.268 Å). Moreover, to form [1 þ 2]-
cycloaddition product 300, the Cipso-N-N-Cipso dihedral angle
(-111.3°) is required to be reduced so as to facilitate H2 elimina-
tion. This configurational alteration induces a steric crowding
between the two o-hydrogens of the phenyl rings in diphenylhy-
drazine fragment, resulting in a strained silicon fused three-mem-
bered ring (300, Figure 1). The explanation justifies that though the
amine hydrogen is more acidic than the aromatic o-hydrogen, the
C-H activation is a preferred route to [1 þ 4]-cycloaddition.
The density functional theory (DFT) optimized structures of the
three spirocyclic compounds (10, 20, and 30) are shown in Figure 1.
The gas-phase optimized structures show a close resemblance with
the X-ray crystal structure data (see Table 1). The exclusive
formation of [1 þ 4]-cycloaddition product 2 rather than the [1
þ 2]-cycloaddition complex 200 can be primarily explained from the
inherent ring strain present in the latter. This is supported by the
fact that 200 is energetically less stable (ΔEe = 38.3 kcal/mol)
compared to 20. A similar but relatively less pronounced effect is
visible in 3, where the [1 þ 2]-cycloaddition product 300 is less
stable (ΔEe = 29.4 kcal/mol) with respect to its isomer 30
(Figure 1). The reaction of L with 1,2-diphenylhydrazine initially
’ CONCLUSION
We have shown three new different routes for the synthesis of
silicon centered spirocyclic compounds (a) by dearomatization
of a pyridine ring, (b) by [1 þ 4]- rather than [1 þ 2]-
cycloaddition product formation involving conversion of azo
compound to hydrazone derivative, and (c) by elimination of
hydrogen. For all three products the reaction occurred at the
silicon atom, and a new five-membered ring was formed, keeping
the SiN2C3 six-membered ring intact. The experimental results
have been supported by DFT calculations. These different
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pathways of reactivity shown by L make the chemistry of silylene
much more fascinating.
NMR (99.36 MHz, C6D6, 25 °C): δ -39.35 ppm. EI-MS: m/z 626.4
(Mþ).
Crystal Structure Determination. Suitable single crystals for
X-ray structural analysis of 1, 2, and 3 were mounted at low temperature
in inert oil under argon atmosphere by applying the X-Temp2 device.23
The data of 1, 2, and 3 (Table 2) were collected on a Bruker D8 three
circle diffractometer equipped with a SMART APEX II CCD detector
and an INCOATEC Mo microsource with INCOATEC Quazar mirror
optics.24 The data were integrated with SAINT,25 and an empirical
absorption correction with SADABS26 was applied. The structures were
solved by direct methods (SHELXS-97) and refined against all data by
full-matrix least-squares methods on F2 (SHELXL-97).27 All non-
hydrogen-atoms were refined with anisotropic displacement parameters.
The hydrogen atoms were refined isotropically on calculated positions
using a riding model with their Uiso values constrained to 1.5Ueq of their
pivot atoms for terminal sp3 carbon atoms and 1.2 times for all other
carbon atoms.
Computational Details. All calculations are performed in Gauss-
ian03 quantum code.28 The geometries of all the compounds are
optimized with the generalized approximation (GGA) to DFT by using
the exchange functional of Becke29 in addition with the correlation
functional of Perdew30 (BP86). All the atoms are treated with Ahlrich0s
split valence plus polarization (SVP) basis sets31 of 6-31G* quality. In all
our DFT treatments, the resolution-of-the-identity (RI) approximation
(also called “density fitting”) for the two electron integrals was
employed.32 The geometries were optimized without any symmetry
constraints. All the figures are made in the Chemcraft visualization
software.33
’ EXPERIMENTAL SECTION
All manipulations were carried out in an inert atmosphere of dinitro-
gen using standard Schlenk techniques and in a dinitrogen filled glove-
box. Solvents were purified by MBRAUN solvent purification system
MB SPS-800. All chemicals were purchased from Aldrich and used
without further purification. L was prepared as reported in the
literature.3c 1H, and 29Si NMR spectra were recorded with a Bruker
Avance DPX 200, Bruker Avance DRX 300, Bruker Avance DRX 400, or
a Bruker Avance DRX 500 spectrometer, using C6D6 as solvent.
Chemical shifts δ are given relative to SiMe4. EI-MS spectra were
obtained using a Finnigan MAT 8230 instrument. Elemental analyses
were performed by the Institut f€ur Anorganische Chemie, Universit€at
G€ottingen. Melting points were measured in a sealed glass tube on a
B€uchi B-540 melting point apparatus.
Synthesis of 1. n-Hexane (60 mL) was added to a 100 mL Schlenk
flask containing L (0.47 g, 1.06 mmol) and benzoylpyridine (0.19 g, 1.04
mmol), and the solution immediately turned in color to burgundy red;
the reaction mixture was stirred at room temperature for 12 h. The
solution was filtered, and the solvent was reduced in vacuo to about
30 mL and stored at 0 °C in a freezer for 12 h to obtain burgundy red
colored single crystals. (0.46 g, 70%). Mp 157-158 °C(decomp).
Elemental analysis (%) calcd for C41H49N3OSi (627): C, 78.42; H,
7.87; N, 6.69. Found: C, 78.79; H, 8.00; N, 6.38. 1H NMR (200 MHz,
C6D6, 25 °C): δ 1.22-1.44 (m, 21H, CH(CH3)2), 1.49 (s, 3H,
NCCH3), 1.61 (d, 3H, J = 7 Hz, CH(CH3)2), 3.30 (m, 2H, J = 7 Hz,
CH(CH3)2), 3.34 (s, 1H, NCCH2), 3.89 (m, 2H, J = 7 Hz, CH(CH3)2),
3.96 (s, 1H, NCCH2), 4.86-4.93 (m, 1H, C5NH4), 5.31 (s, 1H, γ-CH),
5.45-5.53 (m, 1H, C5NH4), 6.13-6.21 (m, 2H, C5NH4), 6.85-7.09
(m, 11 H, C6H5, 2ꢀ iPr2C6H3) ppm. 29Si{1H} NMR (59.63 MHz,
C6D6, 25 °C): δ -53.95 ppm. EI-MS: m/z 627.4 (Mþ).
Synthesis of 2. Toluene (60 mL) was added to a 100 mL Schlenk
flask containing L (0.37 g, 0.83 mmol) and diisopropyl azodicarboxylate
(0.17 g, 0.84 mmol). The reaction mixture was stirred at room
temperature for 12 h. The solution was filtered, and the solvent was
reduced in vacuo to 20 mL and stored in a freezer at -32 °C for a week
to obtain colorless single crystals. (0.36 g, 67%). Mp 167-170 °C
(decomp). Elemental analysis (%) calcd for C37H54N4O4Si (646.39): C,
68.69; H, 8.41; N, 8.66. Found: C, 68.21; H, 8.51; N, 7.96. 1H NMR (200
MHz, C6D6, 25 °C): δ 0.53 (d, 3H, J = 6 Hz, CH(CH3)2), 0.60 (d, 3H, J
= 6 Hz, CH(CH3)2), 1.13-1.51 (m, 33H, CH(CH3)2, NCCH3), 3.34
(s, 1H, NCCH2), 3.64 (m, 4H, CH(CH3)2), 3.89 (s, 1H, NCCH2), 4.42
(m, 1H, J = 6 Hz, CH(CH3)2), 5.09 (m, 1H, J = 6 Hz, CH(CH3)2), 5.27
(s, 1H, γ-CH), 6.97-7.18 (m, 6H, 2 ꢀ iPr2C6H3) ppm; 29Si{1H} NMR
(79.49 MHz, C6D6, 25 °C): δ -53.94 ppm. EI-MS: m/z 646.3 (Mþ).
Synthesis of 3. n-Hexane (60 mL) was added to a 100 mL Schlenk
flask containing L (0.25 g, 0.56 mmol) and 1, 2-diphenylhydrazine (0.11
g, 0.59 mmol). The reaction mixture was stirred at room temperature for
12 h. The solution was filtered, and the solvent was reduced in vacuo to
20 mL and stored in a freezer at -32 °C for 3 days to obtain colorless
single crystals. (0.26 g, 70%). Mp 135-137 °C(decomp). Elemental
analysis (%) calcd for C41H50N4Si (626.95): C, 78.55; H, 8.04; N, 8.94.
Found: C, 78.70; H, 8.15; N, 9.52. 1H NMR (200 MHz, C6D6, 25 °C): δ
0.61 (d, 3H, J = 7 Hz, CH(CH3)2), 0.75-0.82 (2d, 6H, CH(CH3)2),
1.22 (d, 3H, J = 7 Hz, CH(CH3)2), 1.27 (d, 3H, J = 7 Hz, CH(CH3)2),
1.37 (d, 3H, J = 7 Hz, CH(CH3)2), 1.50 (d, 3H, J = 7 Hz, CH(CH3)2),
1.52 (s, 3H, NCCH3), 1.62 (d, 3H, J = 7 Hz, CH(CH3)2), 3.07 (m, 1H,
J = 7 Hz, CH(CH3)2), 3.33 (s, 1H, NCCH2), 3.47 (m, 1H, J = 7 Hz,
CH(CH3)2), 3.92 (m, 1H, J = 7 Hz, CH(CH3)2), 4.02 (s, 1H, NCCH2),
4.21 (m, 1H, J = 7 Hz, CH(CH3)2), 4.68 (s, 1H, NH), 5.40 (s, 1H, γ-
CH), 6.52-7.12 (m, 15 H, C6H4, C6H5, 2 ꢀ iPr2C6H3) ppm. 29Si{1H}
’ ASSOCIATED CONTENT
S
Supporting Information. DFT datas for all compounds
b
mentioned in the main text. CIFs for 1 (CCDC-805652), 2
(CCDC-805653), and 3 (CCDC-805654). This material is
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: hroesky@gwdg.de (H.R.), dstalke@chemie.uni-
goettingen.de (D.S.).
’ ACKNOWLEDGMENT
We are thankful to the Deutsche Forschungsgemeinschaft for
supporting this work. R.A. and G.T. are thankful to the Alexander
von Humboldt Stiftung for research fellowships. D.S. and J.H. are
grateful to the Land Niedersachsen for providing the Catalysis of
Sustainable Synthesis (CaSuS) Ph.D. program. D.K. acknowl-
edges Prof. S. Ramakrishnan and Prof. S. Dattagupta for constant
support and encouragement.
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